Decompose Branch

Decompose Branch

Turn a messy local branch into a sequence of atomic commits ready for review.

When to Use

• Branch has messy/WIP commits before PR
• Reviewer feedback says "hard to follow"
• Splitting a large change into reviewable chunks

Note: Commands below use master as base branch. Replace with your actual base if different.

Phase 1: Analyze

Gather context and identify the distinct logical changes.

# See what commits exist on this branch vs base
git log --oneline master..HEAD

# See the full diff against base
git diff master...HEAD

# See changed files
git diff --stat master...HEAD

Read the diff and classify each modified section into one of these categories:

1. Refactors — Renames, extractions, reorganizations (these go first)
2. Structural changes — New types, new fields, new functions
3. Behavioral changes — Modified control flow, new conditions
4. Bug fixes — Isolated corrections
5. Tests — Often map 1:1 to behavioral changes

Present to user:

## Identified Changes

| # | Change | Files | Depends On |
|---|--------|-------|------------|
| 1 | Extract helper for validation | utils.ts | - |
| 2 | Add retry logic to client | client.ts | 1 |
| 3 | Fix timeout handling | client.ts | 2 |
| 4 | Tests for retry and timeout | client.test.ts | 2, 3 |

Stop and ask: "Does this decomposition look correct, or would you like to reorder, combine, or split changes?" Wait for confirmation before proceeding.

Phase 2: Plan the Decomposition

Determine the commit order based on dependencies.

Dependency rules:

• Refactors before features that depend on them
• Types/interfaces before code that uses them
• Core logic before tests
• Changes to the same function ordered by logical progression

For each commit, define:

• Subject line (imperative mood, max 50 chars)
• Which files/sections to include (with line ranges if helpful)
• Whether it should build/pass lint

Present the plan:

## Commit Plan

1. "Extract validation helper from handler"
   - utils.ts: new validateInput function
   - handler.ts: use new helper
   - Builds: Yes

2. "Add retry logic to client"
   - client.ts: retry wrapper, config
   - Builds: Yes

3. "Fix timeout handling in retry loop"
   - client.ts: timeout check in retry
   - Builds: Yes

4. "Add tests for retry and timeout"
   - client.test.ts: new test cases
   - Builds: Yes

Stop and ask: "Ready to execute this plan?" Wait for confirmation before proceeding.

On atomicity: Prefer commits that build and pass lint. Group inseparable changes (e.g., signature changes with caller updates).

Phase 3: Execute

Create a fresh branch and build commits incrementally.

# Save current branch name
ORIGINAL_BRANCH=$(git branch --show-current)

# Create working branch from base
git checkout -b ${ORIGINAL_BRANCH}-decomposed master

# Bring in all changes unstaged
git merge --squash ${ORIGINAL_BRANCH}
git reset HEAD

Now you have all changes as unstaged modifications. Build commits one at a time:

For each planned commit:

1. Stage only the relevant files/sections
2. Verify it builds (if plan says "Builds: Yes")
3. Commit with the planned message
4. Repeat

# Stage by file (for hunk-level control, edit files to isolate changes first)
git add <file>

# Verify build (required if plan says Builds: Yes)
# Ask user for build command if unknown

# Commit
git commit -m "<planned message>"

After all commits:

# Verify nothing left unstaged
git status

# If leftover changes exist:
# - Amend last commit if changes logically belong there
# - Create cleanup commit if unrelated to existing commits
# - Discard if debug/temp code (git checkout -- <file>)

Phase 4: Verify

Ensure the decomposed branch matches the original.

# Compare final state to original branch (should be empty or whitespace-only)
git diff ${ORIGINAL_BRANCH}..HEAD

If differences exist:

• Acceptable: formatting, whitespace only
• Not acceptable: missing changes — add a commit or amend to include them

Present summary:

## Decomposition Complete

Original: feature-branch (5 messy commits)
Result: feature-branch-decomposed (4 atomic commits)

| # | Commit | Summary |
|---|--------|---------|
| 1 | abc123 | Extract validation helper |
| 2 | def456 | Add retry logic |
| 3 | ghi789 | Fix timeout handling |
| 4 | jkl012 | Add tests |

**Next steps:**
git log --oneline master..HEAD    # Review commits
git diff master..HEAD             # Verify full change
git branch -m feature-branch feature-branch-old  # Backup original
git branch -m feature-branch-decomposed feature-branch  # Rename

HIL Pattern

• Ask for branch name if not on a feature branch
• Stop after Phase 1 and Phase 2 for user approval
• Confirm before any destructive action

Guardrails

• Never modify master
• Never force push without approval
• Keep original branch until user confirms
• If build fails unexpectedly, stop and ask
• Prefer cohesive commits over fragmentation